As a transmission mode of digital data showing various signal information, an optical transmission system which converts digital data to an optical signal and transmits via transmission path formed by using optical fiber has been proposed. The digital data to be processed under such optical transmission system is serial data having the bit rate of approximately from several hundreds (Mbps) to one (Gbps).
The digital data to be processed under the optical transmission system is converted from 8-bit word string data in which 1 word is formed with 8 bits into 10 bits word string data in which 1 word is formed with 10 bits, that is 8-10 bit conversion is conducted at the transmitting end and transmitted as 10-bit word string data. Then, at the receiving end, 10-8 bit word string data conversion that converts the 10-bit word string data received to the former 8-bit word string data is conducted.
Every word having 10 bits in the 10-bit word string data used in the transmission system is categorized in terms of the number of ones and zeros; i.e., the number of ones is larger than the number of zeros, the number of zeros is larger than the number of ones, and the equal numbers of ones and zeros are present. In this connection, in order to indicate the condition of the number of ones and zeros, the idea of running disparity (RD) is adopted; it is called that when the number of ones is larger than the number of zeros, the running disparity (RD) is plus, and when the number of zeros is larger than the number of ones, the RD is minus, and moreover, when the number of ones and the number of zeros are the same, the RD is neutral. And the word data having more ones than zeros is called as the word data having plus RD (plus word data), the word data having more zeros than ones is called as the word data having minus RD (minus word data), and the word data having the equal number of ones and zeros is called as the word data having neutral RD (neutral word data).
Furthermore, in the case of transmitting 10-bit word string data, considering the word synchronization that divides serial data per 10-bit word correctly at the receiving end, word synchronous data will be inserted as required. This word synchronous data is the word consisting of 10 bits but this word is the word having special code that can not be used as the 10-bit word to conduct the information transmission. In the case where the word synchronous data is being assigned, if the immediately preceding word data is the data having minus RD, it is considered that the word synchronous data has plus RD, and if the immediately preceding word data has plus RD, it is considered that the word synchronous data has minus RD.
FIG. 10 shows an example of 10-bit word synchronous data DS (10), and when the immediately preceding word data has minus (-) RD, i.e., CRD (current running disparity), the word synchronous data DS (10) is considered as "001111 1010" having plus RD, and when the immediately preceding word data has plus (+) CRD, the word synchronous data DS (10) is considered as "110000 0101" having minus RD (hereinafter "001111 1010" is referred to as the word synchronous data +DS (10), and "110000 0101" is referred to as the word synchronous data -DS (10)).
These examples of word synchronous data "001111 1010" and "110000 0101" are defined as the code name K28.5 based on the fiber channel standard ANSI X3.230-1994.
When the 10-bit word string data to be transmitted under the above condition is regarded as the data showing image signal information, it is considered that 10-bit word can be correctly divided using the word synchronous data at each horizontal interval at the receiving end by inserting word synchronous data at each horizontal interval of the image signal.
In this case, either the word synchronous data +DS (10) having plus RD or the word synchronous data -DS (10) having minus RD will be inserted at each horizontal interval of image signal depending upon whether the RD of the immediately preceding word data is minus or plus at the transmitting end.
For example, in the case of inserting the word synchronous data at the transmitting end, if the RD of the immediately preceding word data is minus, plus word synchronous data +DS (10) is inserted, and when the RD of the immediately preceding word data is plus, minus word synchronous data -DS (10) is inserted.
The image signal information in which word synchronous data is inserted is converted from 20-bit parallel data to serial data and transmitted from the transmitting end to the receiving end, and word synchronous data is extracted from the 10-bit word string data at the receiving end, and thus dividing the serial transmission data per 10-bit word, it is considered that the serial data can be correctly divided per 10-bit word at each horizontal interval in which said word synchronous data is inserted.
In the case of detecting the word synchronous data, the receiving end that sequentially receives 10-bit word string data of the image signal information (serial data) to which the word synchronous data +DS (10) or -DS (10) has been assigned at each horizontal interval of the image signal information at the transmitting end, only detects plus word synchronous data +DS (10) but does not detect minus word synchronous data -DS (10).
However, if the word synchronous data detection at the receiving end only detects the word synchronous data +DS (10) and not detect the word synchronous data -DS (10), there causes an inconvenience in conducting the data processing at the receiving end, since either the word synchronous data having plus RD +DS (10) or having minus RD -DS (10) is inserted depending upon the RD condition of the immediately preceding word data, i.e., the RD of the immediately preceding data is minus or plus, at each part corresponding to the horizontal synchronization in image signal.
More specifically, since the receiving end only detects the word synchronous data +DS (10) and does not detect the word synchronous data -DS (10) when conducting the processing of 10-bit word string data, the condition in which word synchronous data detection can be conducted at the part corresponding to the horizontal interval in the image signal information can not be secured, and as a result, when reproducing the image signal information represented by 10-bit word string data, it caused a problem that the correct word synchronization at each horizontal interval could not be obtained.